1. Introduction
This invention relates to electromagnetic containment using shielding comprising a non-conducting substrate coated with multiple coatings of an autocatalytic electroless metal alloy of copper, phosphorus and a member from the group of cobalt, nickel and mixtures of cobalt and nickel and to processes for making of the same.
2. Description of the Prior Art
Electromagnetic interference (EMI) radiation is undesirable energy emissions within a frequency range of from less than 60 Hz to more than 1,000 MHz. Radio frequency interference (RFI) is the portion of EMI radiation in about the 0.01 to 1,000 MHz range.
EMI radiation is created by operation of many diverse forms of electronic equipment ranging from microwave equipment to home computers. The radiation occurs because electronic devices emit "noise" in the above frequency range that is picked up by other devices or by conduction through power lines that act as antennas. EMI radiation may interfere with other devices and has been known to cause such diverse problems as interference with police mobile radios, communication systems, scientific test equipment, and cardiac pacemakers.
In recent years, the use of electronic equipment in the home and work place has grown rapidly; correspondingly EMI emissions have increased. Additionally, most housings for electronic equipment are now fabricated from plastic rather than metal. Plastics are lighter, more versatile, and less expensive than metal; however, they do not possess the intrinsic EMI/RFI shielding capabilities provided by metal enclosures.
The Federal Communications Commission (FCC) has published a series of regulations concerning standards for maximum allowable EMI emissions for electronic devices. The regulations, which became effective in October 1983, apply to all digital electronic products that use or generate frequencies between 10 KHz and 1,000 MHz. These regulations therefore include commercial, industrial, business, and home products such as computers, calculators, cash registers, electronic typewriters, video equipment, and electronic games. The regulations require that the electronics industry develop electronic devices which have electromagnetic compatibility (EMC); in other words, equipment which neither interferes with other devices nor is itself susceptible to interference.
One approach to limiting electromagnetic containment is the use of an EMI shield to contain the radiation. Containment requires special shielding materials, components, and structures which prevent generated energy from escaping and acting as a source of disturbance.
The effectiveness of electromagnetic containment is determined by the degree to which the field strength is attenuated as a result of reflection or absorption by the shielding material. Shielding efficiency (SE) is calculated as a logarithmic function of the ratio of unshielded EMI transmission to shielded EMI transmission and is expressed in decibels. Because of the logarithmic nature, an increase of 30 decibels in SE for a given wavelength or frequency of electromagnetic radiation represents a 1,000 percent increase in the shielding efficiency of the coating. A coating with a shielding efficiency of 30 db, for example, eliminates 99.9% of the total EMI radiation. A 60 db coating eliminates 99.9999% of the total EMI radiation.
There are several shielding methods in use. The method most often used involves a metallic coating applied over a plastic housing for the electronic device. An early publication showing the use of multiple metal coatings over a plastic housing is Engineering, 9, December, 1966, pp. 1026 and 1027. Methods for applying metallic coatings disclosed in this reference include galvanic deposition, spray coating, chemical metalizing and vacuum metalizing. Metal coatings include copper, silver, chromium, nickel, silver, gold, zinc, etc.
Metals are applied over housings for electronic equipment in a number of ways. For example, EMI shielding materials have been arc-sprayed (zinc) and painted with metal-containing paints (nickel) onto the electronic housings. Both of these methods have serious disadvantages. Arc-sprayed zinc is an effective EMI shield with attenuation to 120 db or more. However, zinc is toxic and expensive, the procedure is labor intensive, and the coating is prone to cracking and peeling. Conductive nickel paints are easier to apply than arc-sprayed coatings, but do not cover recessed areas, provide attenuation to only 20 to 60 dbs and often require multiple coatings.
Silver and copper conductive paints have also been used in the manufacture of EMI shielding. Silver is a good conductor, but is expensive and oxidizes. Copper conductive paints are easy to apply, economical, used with conventional equipment, are wear resistant and have good resistance to flaking. However, copper tends to oxidize which results in a loss of conductivity and a concomitant loss of shielding effectiveness.
Other methods for applying metallic coating include cathode sputtering and vacuum metalizing. Such coatings show good conductivity and good adhesion, but require expensive equipment for application, are prone to microscopic cracking, can distort thermoplastics, require high power, are batch operations and are limited by part configuration.
Recently, interest has been generated in the use of electroless metals for EMI shielding. Electroless plating of surfaces for EMI shielding is shown in the prior art as early as 1967. Lordi, Plating, 54, 382, (1967), incorporated herein by reference, discusses the use of both electroless copper and electroless nickel as shielding materials. Lordi discusses electronic applications for electroless copper and electroless nickel specifically noting EMI shielding, teaches that electroless nickel can be used as an intermediate coating over copper to prevent corrosion and finally, that electroless copper can be protected by a coating of a second metal to prevent oxidation.
Recently, a number of publications have discussed the use of electroless metals for EMI applications. Plastics Technology, Vol. 27, June '81, p. 67, teaches the use of electroless metals as EMI shielding materials. Plastics World, Vol. 40, pp. 40-45, September 1982 states that electroless plating may be less expensive than many of the shielding processes now in use and can give comparable shielding performances. The economy of application of electroless plating for EMI shielding is demonstrated in a 1982 article in Industrial Finishing, Vol. 58, pp. 100 to 101. Smoluk, Modern Plastics, September '82, pp.48-51 cites several commercially available electroless plating systems for shielding applications. Smoluk reports electroless copper coatings with demonstrated SE values of 80 to 116 db, and electroless nickel coatings with SE values exceeding 45 db.
As discussed in the literature, both electroless copper and electroless nickel have been used in the electroless plating of plastic substrates. Both have advantages and disadvantages. Copper, with a relative conductivity of 1.0 (second only to silver with a conductivity of 1.05), has high shielding effectiveness. An additional advantage of copper is a relatively low cost. Disadvantages of copper are relatively low abrasion (wear) resistance and a relatively poor corrosion resistance with a strong tendency to oxidize which significantly reduces the shielding effectiveness.
Electroless nickel serves as a good paint base, has high wear resistance, stable electrical contact resistance, good solderability, and good corrosion resistance. Plastic Design Forum, November/December 1982, pp. 17-26, states that while electroless nickel is less conductive than electroless copper and therefore less effective as a shielding material, it possesses better corrosion resistance and may be preferable to electroless copper for EMI shielding applications, especially in severe environmental conditions. The major disadvantage of electroless nickel is its low relative conductivity of 0.20 or less. However, electroless nickel is relatively expensive and therefore, high cost is a disadvantage to the use of electroless nickel as a shielding material.
Disadvantages attendant to the use of electroless copper and electroless nickel separately as shielding materials are partly overcome by a dual layer of electroless copper overplated with electroless nickel. Such a dual layer is believed to be first suggested by Lordi (supra) in 1967. In 1983, Krulik, in Industrial Finishing, May, 1983, pp. 16-18, states that "the (electroless) copper's disadvantages are overcome by coating the copper layer with a thin layer of electroless nickel. The electroless nickel is deposited to protect the copper. The nickel's relative high cost is minimized by the thinness of the layer." A 1983 article by Hadju and Krulik in Plating and Surface Finishing, July, 1983, pp. 42-44, states that "a composite coating of electroless copper with a top layer of electroless nickel will combine the desirable characteristics of both. There is no degradation of the excellent shielding properties of electroless copper which can be adjusted in its shielding efficiency by varying its thickness. A relatively thin coating of electroless nickel provides corrosion resistance, paint adhesion, stable low electrical contact resistance, and other desirable properties and may be maintained at a constant thickness". A dual layer of electroless copper coated with electroless nickel is also disclosed in U.S. Pat. No. 4,514,486 incorporated herein by reference. This configuration utilizes the high conductivity of the electroless copper for EMI attenuation and the corrosion resistance of the electroless nickel to protect the copper aqainst oxidation.
Processes and compositions for plating electroless nickel over copper surfaces including electroless copper surfaces are known in the prior art and are described in numerous publications including U.S. Pat. No. 4,002,778 incorporated herein by reference. This patent teaches a process for plating electroless nickel onto copper circuit boards by pretreating the materials with a reducing agent such as sodium borohydride prior to chemical plating. Another patent involving multiple electroless metal coatings is U.S. Pat. No. 4,169,171 incorporated herein by reference. This patent teaches the application of electroless nickel over electroless copper for bright and decorative coatings. Gallis, Plastic Design Forum, Novomber/December 1983, states that in electroless plating, the usual routine is copper followed by a flash of a very thin layer of a nickel topcoat. In this article, Gallis also discloses a recently introduced electroless copper and electroless nickel system which incorporates optional, corrosion resistant topcoats to achieve the specific surface properties desired.
Briefly, electroless plating of plastics comprises immersing a part in a series of baths which prepare the surface of the part for deposition of an adherent metallic layer on the surface of the part. Following preparation of the part, it is immersed in an electroless plating solution where plating takes place by chemical reduction of a dissolved metal in the presence of a catalytic surface.
Conventional methods of electroless plating onto dielectric substrates require that the surface be catalyzed prior to deposition of the metallic coating. Colloidal solutions containing noble metals have been used in electroless plating systems to render non-conductive surfaces catalytic to deposition of the dissolved metal. An example of a noble metal catalyst is disclosed in U.S. Pat. No. 3,011,920 incorporated herein by reference. The patent teaches treatment of the dielectric substrate with a colloidal palladium solution to render it catalytic to deposition of the dissolved metal.
Conductive surfaces such as metallic surfaces also may be plated with electroless metal deposits. Though conductive, most metal surfaces are not catalytic to electroless metal deposition and typically, such surfaces must be activated before deposition from an electroless solution may be initiated. In similar fashion, electroless metal deposits are not catalytic to a second electroless deposit. For example, electroless copper must be activated to electrolessly deposit a second electroless metal such as electroless nickel. Consequently, if one desires to form a dual layer of electroless copper coated with electroless nickel, such as in the manufacture of EMI shielding, it is necessary to activate the electroless copper prior to deposition of electroless nickel from an alkaline hypophosphite solution. This increases the processing line and the costs of manufacture.